This invention relates to an arc welder used for a.c. arc welding.
Generally, in a.c. arc welding, a current flowing through a welding load alternates its polarity and undergoes its zero value every half of its alternating period, thereby interrupting an arcing state. Therefore, it is necessary to recover the arcing state every such time. When a base material forming the welding load between a welding electrode is a metal such as aluminium, which is easily oxidized, and an a.c. current of a sine waveform is supplied to the welding load an oxide film on the surface of the base material is removed by cleaning phenomena during a half period for which the base material is negative and the electrode is positive, but it becomes difficult to recover the arcing state since electron emission is retarded by lacking energy directly after polarity transition. In a prior art arc welder used for arc welding of this kind, therefore, its arcing recovery power has been improved by converting the sine waveform of the welding current into a square waveform.
Such arc welder of square waveform type is disclosed, for example, in the Japanese utility model publication gazette No. H3(91)-41891. In this welder, as shown in FIG. 1, a commercial three-phase a.c. power supplied to power input terminals 1a, 1b and 1c is rectified and smoothed by a three-phase input rectifier 2 of diode-bridge configuration and a smoothing capacitor 3 and then applied to a high frequency a.c. convertor device 4. The device 4 is composed of a full bridge invertor including switching elements such as insulated-gate bipolar transistors (hereinunder referred to as "IGBTs") and field-effect transistors (hereinunder referred to as "FETs") and controlled by a high frequency invertor control device 5 to convert the input d.c. current into a high frequency a.c. current. The high frequency a.c. output of the device 4 is supplied to a primary winding 6a of a main transformer 6 and the high frequency a.c. output of its secondary winding 6b is rectified by a main rectifier 7 of diode-bridge configuration. The d.c. output of the rectifier 7 across its positive and negative output terminals 7p and 7n is smoothed by a d.c. reactor 8 and supplied to a low frequency a.c. convertor device 9. The device 9 is composed of a full-bridge invertor including switching elements 10, 11, 12 and 13 such as IGBTs or FETs and controlled by a low frequency invertor control device 14 to convert the input d.c. current into a low frequency square waveform a.c. current of ten to several hundred Hertz which is then supplied through output terminals 9a and 9b to an electrode 20 and a base material 21 which form a welding load 19 therebetween. The switching elements 10, 11, 12 and 13 are provided with back-current preventing diodes 15, 16, 17 and 18 respectively connected in parallel thereto.
In FIG. 2, A denotes a waveform of the output current flowing through the welding load 19. As shown, the welding current in this case is an alternating current having a square waveform, which has a large energy at the time of polarity transition and improves the arcing recovery power. In the drawing, T.sub.en denotes a normal polarity period for which the base material 21 is positive and the electrode 20 is negative and T.sub.ep denotes an inverse polarity period for which the base material 21 is negative and the electrode 20 is positive. However, in this case also, when the welding current is below 50 amperes, recovery of the arcing state is liable to become difficult especially at the time of normal-to-inverse polarity transition, since the energy stored in the d.c. reactor is insufficient.
In order to compensate for this shortcoming, the device of FIG. 1 is provided with an auxiliary d.c. power supply 22 used for arcing recovery voltage superposition as shown. The d.c. power supply 22 includes a boosting tertiary winding 6c of the main transformer 6, an auxiliary rectifier 24 and a capacitor 25 and an a.c. output of the tertiary winding 6c of the main transformer 6 is rectified and smoothed by the rectifier 24 and the capacitor 25 to produce an auxiliary arcing recovery voltage which is higher than the main d.c. voltage. The auxiliary voltage is supplied through a current limiting resistor 26 to the input of the low frequency a.c. convertor device 9 and superposed upon the main d.c. voltage. The auxiliary voltage serves to raise a no-load voltage at the time of polarity transition of the welding load 19 and facilitates recovery of the arcing state to remove the above-mentioned shortcoming.
A diode 27 connected in parallel to the current limiting resistor 26 is used for absorbing a transient voltage and serves to prevent transient rise of the input voltage of the low frequency a.c. convertor device 9. More particularly, when a cable for connecting the output terminals 9a and 9b of the device 9 to the welding load 19 is substantially long and its inductance is not negligible, an excessive transient voltage due to this inductance is liable to occur at the input of the device 9 at the instant of polarity transition of the welding load 19 based upon the switching operation of the switching elements 10 to 13. At this time, however, the diode 27 conducts to have the capacitor 25 absorb the transient voltage, thereby suppressing the voltage rise at the input of the device 9.
In the above-mentioned prior art arc welder, the auxiliary d.c. power supply 22 is always connected through the current limiting resistor 26 to the input of the low frequency a.c. convertor device 9. On the other hand, it has been confirmed experimentally that the no-load voltage between the welding electrode 20 and the base material 21 must be 200 volts at least for recovering the arcing state and it must be 250 volts for assuring stable arcing recovery power. Since the d.c. current in the current limiting resistor 26 is five amperes and the arcing voltage between the electrode 20 and the base material 21 is twenty volts in a general welding condition, a power loss in the current limiting resistor 26 reaches (250-20).times.5=1150 watts if the no-load voltage is preset to 250 volts. Therefore, the current limiting resistor 26 must be of large capacity and large size and the transformer 6 and the rectifier 24 must have a constitution of large capacity and size, too. Accordingly, it retards the plan of size and weight reduction of the arc welder.
Accordingly, an object of this invention is to provide an arc welder which can be reduced in both size and weight by reducing the power loss at the time of superposition of the auxiliary arcing recovery voltage from the auxiliary power supply with the main voltage, while maintaining its high arcing recovery power.
Another object of this invention is to absorb the excessive transient voltage occurring in the low frequency a.c. convertor device into the auxiliary d.c. power supply and also suppress the auxiliary voltage rise following this absorption.